1. Field of the Invention
The invention relates to imaging devices and, in particular, to color image sensors.
2. Description Relative to the Prior Art
Color image sensors of various types have been proposed for and used in video cameras. To avoid optical complexity and problems with image registration, it is highly desirable that color image sensing occur at a single imaging site, e.g., at a single planar photosensitive array. Difficulty is encountered with such "single-site" color imaging, however, because at least three distinct types of color information must be extracted in order to represent a color image in video signal form.
One known approach to providing a single-site color sensing device utilizes a single image sensor of broad wavelength sensitivity and a cooperating filter disc which passes a series of color filters through the image beam in a repeating sequence. The filter interpositions are synchronized to image scanning, a filter typically being interposed during an entire field scan. Devices operating in this manner are said to produce a "field sequential" color signal. One problem with this approach is that the resulting signal presents the extracted color image information in a time order which is radically different from the time order of the standard NTSC video signal. A further disadvantage is that some of the color image information (e.g., blue image information if a blue basic color vector is utilized) tends to be disproportionately detailed and hence wasteful of sensor capacity in consideration of the response characteristics of the human visual system.
Certain other proposed approaches to achieving single-site color image sensing call for the use of striped color filters superposed on a single image sensor. One such type of image sensor utilizes filter grids which are angularly superimposed on one another (see U.S. Pat. No. 3,378,633). As a result of image scanning, such image sensors produce a composite signal wherein chrominance information is represented in the form of modulated carrier signals. Such apparatus may be adapted to produce signals in the NTSC format or, if desired, the color image information can be separated by frequency domain techniques. In practice, however, it has proven difficult to produce such sensors economically, particularly where detailed image information is required.
Striped filters which transmit a repeating sequence of three or more spectral bands have also been proposed for use in color imaging. With this arrangement, the filters are typically aligned vertically and scanning of the image is performed horizontally. In effect, elemental sample areas are defined along the filter stripes. With this arrangement, it will be appreciated, sampling for a given color is not uniform for horizontal and vertical directions. Additionally, the sampling patterns which result tend to provide a disproportionate quantity of information regarding basic color vectors to which the eye has less resolving power, e.g., "blue" information relative to "green" information.
Another approach to color imaging which has been proposed is the "dot" scanning system, as discussed in U.S. Pat. No. 2,683,769 to Banning. That approach generally utilizes spectrally selective sensor elements which are arranged in triads (red, green, and blue elements, respectively). However, in U.S. Pat. No. 2,755,334, also to Banning, a repeated arrangement of four element groupings (red-, green-, blue-, and white-sensitive elements, respectively) is described. Such approaches to color imaging have not been of practical significance, in part because of the cost of fabricating the number of individual elements which are required to provide image information having adequate detail.
In summary, while color imaging devices having a single imaging site are desirable to minimize optical and registration problems and to provide a more rugged camera structure, video camera manufacturers generally resort to splitting the image beam and providing multiple image scanners in order to achieve a satisfactory type and quality of color video signal.